What causes "Usage $0 " in GoogleChrome/lighthouse?

Run the Lantern collection/computation step first to produce the computed results JSON at INPUT_PATH.

Usage $0 <computed file>

Error message

Usage $0 <computed file>

What it means

Thrown at the top level of assert-baseline-lantern-values-unchanged.js when either the HEAD computed-results file (INPUT_PATH) or the BASELINE_COMPUTED_PATH does not exist on disk. The script compares current Lantern metric values against a checked-in baseline, so both files must be present.

Source

Thrown at core/scripts/lantern/assert-baseline-lantern-values-unchanged.js:23

 * SPDX-License-Identifier: Apache-2.0
 */

import fs from 'fs';
import assert from 'assert/strict';
import path from 'path';

import chalk from 'chalk';

import constants from './constants.js';
import {readJson} from '../../test/test-utils.js';

const INPUT_PATH = process.argv[2] || constants.SITE_INDEX_WITH_GOLDEN_WITH_COMPUTED_PATH;
const HEAD_PATH = path.resolve(process.cwd(), INPUT_PATH);
const BASELINE_PATH = constants.BASELINE_COMPUTED_PATH;
const BASELINE_ACCURACY_PATH = constants.BASELINE_ACCURACY_PATH;

if (!fs.existsSync(HEAD_PATH) || !fs.existsSync(BASELINE_PATH)) {
  throw new Error('Usage $0 <computed file>');
}

const computedResults = readJson(HEAD_PATH);
const expectedResults = readJson(BASELINE_PATH);

/** @type {Array<{url: string, maxDiff: number, diffsForSite: Array<DiffForSite>}>} */
const diffs = [];
for (const entry of computedResults.sites) {
  // @ts-expect-error - over-aggressive implicit any on candidate
  const expectedLantern = expectedResults.sites.find(candidate => entry.url === candidate.url);
  const actualLantern = entry.lantern;

  let maxDiff = 0;
  /** @type {DiffForSite[]} */
  const diffsForSite = [];
  Object.keys(actualLantern).forEach(metricName => {
    if (!(metricName in expectedLantern)) throw new Error(`missing metric ${metricName}`);

View on GitHub (pinned to 9515cd4e58)

Solutions

Run the Lantern collection/computation step first to produce the computed results JSON at INPUT_PATH.
Verify BASELINE_COMPUTED_PATH (from constants.js) exists in the repo; if missing, regenerate or restore from git.
Pass the correct computed-file path as the first argument, or omit it to use the default.

Example fix

// before
node core/scripts/lantern/assert-baseline-lantern-values-unchanged.js wrong-path.json
// after
npm run lantern-collect && node core/scripts/lantern/assert-baseline-lantern-values-unchanged.js

Defensive patterns

Strategy: validation

Validate before calling

const fs = require('fs');
function assertInputsExist(headPath, baselinePath) {
  if (!fs.existsSync(headPath) || !fs.existsSync(baselinePath)) {
    console.error('Usage: $0 <computed file>');
    process.exit(1);
  }
}

Prevention

Run the computation/generation step before the assertion script.
Verify the baseline file is present in the repo (git checkout if missing).
Pass the correct path argument or rely on the default constant.

When it happens

Trigger: Running the baseline assertion script before generating the computed results file, or before the baseline has been created; passing a wrong path argument.

Common situations: Running the script on a fresh checkout without building results first; CI job ordering wrong (assertion before generation); typo in the path argument.

Related errors

AI-assisted analysis of GoogleChrome/lighthouse@9515cd4e58 (2026-08-13). Data as JSON: /api/errors/48beb09c478352fa. Report an issue: GitHub.